This invention relates to a camshaft drive for an engine and more particularly to an improved camshaft drive arrangement for an overhead camshaft that permits a compact engine construction.
In substantially all engine applications, there is a large demand for more compact engine constructions. This is particularly true in connection with automotive applications. One particularly demanding situation in an automotive application is when the engine is positioned transversely in the chassis. Such transverse placement normally substantially limits the number of cylinders that can be used because of the length constraints.
Where overhead camshafts are employed, the camshafts are normally driven off of the crankshaft by a timing mechanism that is generally provided at one end of the crankshaft. Such arrangements have a number of difficulties, not the least of which is the fact that this type of a drive generally adds to the overall length of the engine.
There has been proposed, therefore, a type of camshaft drive where the camshaft is driven from an intermediate shaft which is in turn driven from the crankshaft. By doing this, it is possible to shorten the length of the crankshaft and, accordingly, the entire engine. In addition, since there are several stages in the drive from the crankshaft to the camshaft, it is possible to utilize smaller driving sprockets or pulleys in order to achieve the necessary two-to-one speed reduction between the speed rotation of the crankshaft and the camshaft. This further facilitates the formation of compact engine constructions.
An example of an engine having this type of construction may be found in U.S. Pat. No. 5,154,144 issued Oct. 13, 1992, and assigned to the assignee hereof.
Although this type of structure is quite advantageous, the intermediate shaft is mounted in the cylinder head and this tends to give rise to a rather bulky cylinder head arrangement. This may be best understood by reference to FIG. 1, which is a view showing this prior art type of engine camshaft driving arrangement. The arrangement shown in FIG. 1 is not exactly the same as that shown in the aforenoted issued United States Letters Patent, but the problems presented by it are evident from this figure.
As seen in this figure, an engine, shown partially with portions removed and other portions broken away, indicated generally by the reference numeral 11, is comprised of a cylinder block assembly 12 to which a cylinder head assembly 13 is detachably affixed in a known manner. The cylinder head assembly 13 includes a main cylinder head member 14.
In the illustrated prior art example, the engine 11 is of a live-cylinder, inline type and thus the cylinder block 12 is formed with five, inline cylinder bores. Pistons, which are not shown, are slidably supported in the cylinder bores and are connected to a crankshaft that is rotatably journaled in a crankcase assembly by connecting rods, none of which structure is illustrated inasmuch as it is well known.
A first intermediate shaft 15 is journaled within a chamber formed at one side of the cylinder block and which has an exterior surface 16 that receives a closure plate so as to enclose this chamber. This first intermediate shaft has a gear 17 formed on it that is in mesh with a gear formed on the crankshaft at the throw between the first and second cylinders. This gear arrangement preferably drives the first intermediate shaft 15 at the same speed as the crankshaft. However, because of the gear drive arrangement, the first intermediate shaft 15 will rotate in an opposite direction from that of the crankshaft. Therefore, there are balance masses 18 and 19 formed on this first intermediate shaft 15 so as to balance some of the unbalanced forces in the engine.
The first intermediate shaft 15 is journaled by bearing surfaces formed in web portions 21 of the surface 16 and by the cover plate attached thereto.
A driving sprocket 22 is affixed on the first intermediate shaft 15 adjacent the gear 17. A chain 23 or other flexible transmitter drive extends upwardly from the sprocket 22 and is entrained with a further sprocket 24 affixed to one end of a second intermediate shaft 25. This second intermediate shaft 25 is journaled in the cylinder head member 14 by suitable bearing surfaces. It should be noted that the sprocket 24 is disposed between the intake port openings 26 of the number 1 and number 2 cylinders.
At least one overhead camshaft 27 is journaled in the cylinder head assembly 13 and has cam lobes 28 for operating valves associated with the respective cylinders. This camshaft has a sprocket 29 affixed to its forward end and which is driven by a sprocket 31 affixed to the forward end of the second intermediate shaft 25 through a flexible transmitter such as a chain 32.
The combined ratio between the sprockets 22 and 24 and 31 and 29 is two-to-one so that the camshaft 27 will be driven at one-half crankshaft speed. Because the reduction need not take place in a single stage, the sprockets 22, 24, 31 and 29 can be smaller than if the reduction was made in a single stage. This permits a more compact engine and since the drive sprocket for the camshaft need not be formed at an end of the crankshaft, the engine can be made shorter in an overall length.
In spite of these advantages, because of the necessity of mounting the second intermediate shaft 25 in the cylinder head, the cylinder head construction becomes rather complicated and costly. This makes the upper portion of the engine more difficult to position in an engine compartment. This is particularly true when low, sloping hood lines are desired.
Therefore, it is the principal object of this invention to provide an improved camshaft driving arrangement that permits the formation of the compact engine but which nevertheless can be simple and use a more conventional type of cylinder head construction.
It is a still further object of this invention to provide an improved and compact cylinder head, cylinder block and camshaft drive arrangement for an internal combustion engine when the camshafts are driven through a pair of intermediate shafts, but neither of them need be mounted in the cylinder head assembly.